python: fix tracing after Python 3 refactor
python: fix tracing after Python 3 refactor

gem5.opt --debug-flags ExecAll

was failing with:

ImportError: No module named defines

on Python 2.

Change-Id: I06959d6e0ccb7c661c7c749af3392632dac779f3
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18108
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5